The structure of an organic conjugated molecule comprises a conjugated system consisting of delocalized π electrons, thereby presenting special optical, electric and magnetic properties, etc. which catches a wide attention of scientists and has become the focus of studies of the last 20 years. Synthesis based on organic conjugated molecules and functionalization and instrumentalization studies involve many kinds of disciplines such as chemistry, physics, electronics, material sciences, etc. They are multidisciplinary frontiers, filled with vigor and opportunities, and are among one of the important directions of future development of chemistry.
Due to their characteristics of lightness, thinness and flexibility, readiness for modification, etc., organic conjugated molecules have broad prospects of application in the field of photoelectric material. A series of remarkable results have already been obtained, especially in the fields of organic solar cell (OPV), organic light emitting diode (OLED) and organic field effect transistor (OFET), etc. Furthermore, since the organic field effect transistors have the characteristics of readiness for processing, low cost, capacity of large scale flexible preparation, readiness for integration, etc., present obvious advantages in studies in the fields of electronic paper, electronic label, active matrix addressing, sensor and storage, etc., and are considered as having great market potentials.
The organic field effect transistor is an active device regulating the electric circuit in an organic semiconductor by electric field. Its major device structure comprises the 4 following classes: (1) bottom gate bottom contact (BG/BC); (2) top gate bottom contact (TG/BC); (3) bottom gate top contact (BG/TC); and (4) top gate top contact (TG/TC) (Di, C. A.; Liu, Y. Q.; Yu, G.; Zhu, D. B. Acc. Chem. Res., 2009, 42, 1573). The organic field effect transistor consists essentially of electrodes, a dielectric layer, and an organic semiconductor layer, etc. It is essentially a capacitor carrying mobile charges. By applying a voltage between the gate electrode and the source electrode/drain electrode, charges will be induced at the interface between the semiconductor layer and the dielectric layer. When a small voltage is applied between the two electrodes, i.e., the source electrode and the drain electrode, an electric current is formed in the channel. Therefore, the magnitude of the induced charge at the interface can be controlled by adjusting the magnitude of the gate electrode voltage to achieve the on/off of the device, and the amplification of the signal is achieved by controlling the magnitude of the electric current by the voltage between the source electrode and the drain electrode.
The core of the organic field effect transistor is the organic semiconductor layer. The organic semiconductor layer can be classified into p type materials (transporting holes) and n type materials (transporting electrons) based on the difference of the carrier transported in the material; and it can also be classified into organic small molecular materials and organic conjugated polymer materials based on the difference of the type of the organic conjugated molecules. The organic conjugated polymer has been highly regarded because it enables the preparation of the device at a large scale with low cost by solution processing.
The studies on the p type polymer semiconductor materials were initially concentrated on polythiophene systems. The mobility of a sterically regular poly(3-hexylthiophene) (P3HT) can reach 0.05-0.2 cm2V−1s−1 (Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig. P.; de Leeuw, D. M. Nature, 1999, 401, 685). Thereafter, more molecular construction units entered the radar screen of the researchers. These new structures conferred new vigor into this research area. For example, a mobility of 0.94 cm2V−1s−1 was obtained for an organic conjugated polymer based on diketo-pyrrolo-pyrrole (DPP) in 2010 (Li, Y.; Singh, S. P.; Sonar, P. Adv. Mater., 2010, 22, 4862). In 2011, Bronstein reported that a mobility up to 1.94 cm2V−1s−1 was obtained for a DPP-based polymer by different way of connection of the same construction blocks (Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272). A compound obtained by the copolymerization of DPP and thiophene presented a mobility of 0.97 cm2V−1s−1. By a structural modification that used biselenophene to replace bithiophene, a mobility of up to 1.5 cm2V−1s−1 was obtained (Ha, J. S., Kim, K. H., Choi, D. H. J. Am. Chem. Soc. 2011, 133, 10364). Isoindigo type molecules are a family of molecules of significance in addition to DPP. In 2011, we reported that a mobility of 0.79 cm2V−1s−1 and a device stability under high humidity up to 3 months were obtained for a polymer based on isoindigo structures (Lei, T.; Cao, Y.; Fan, Y.; Liu, C. J.; Yuan, S. C.; Pei, J. J. Am. Chem. Soc. 2011, 133, 6099).
In contrast, the development of n type polymer semiconductors is relatively slow. Among them, Facchetti and Marks reported that an electron mobility of 0.01 cm2V−1s−1 was obtained for a polymer based on thiophene and fluorobenzene (Letizia, J. A.; Facchetti, A.; Stern, C. L.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 13476). Zhan et al. reported that a copolymer based on perylene diimide and dithienothiophene exhibited a good field effect performance and its electron mobility can reach 0.013 cm2V−1s−1 (Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.; Li. Y.; Zhu, D.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2007, 129, 7246). Moreover, a naphthalenedicarboximide based polymer reported by Facchetti in 2009 exhibited an electron mobility up to 0.85 cm2V−1s−1 (Chen, Z.; Zheng, Y.; Yan, H.; Facchetti, A. J. Am. Chem. Soc. 2009, 131, 8).
Compared to the traditional silicon solar cells, the organic solar cells have the advantages of low cost, light weight, simple processing, readiness for large scale preparation and readiness for preparing flexible devices, etc. The structure of a device of an organic heterojunction solar cell is primarily classified into two types: one is the forward cell and the other is the reverse cell. The forward cell consists of an anode (generally ITO glass), a hole transport layer (generally PEDOT:PSS), active layer (composed of organic molecular such as organic conjugated polymers and fullerene derivatives, etc.), an electron transport layer, and a cathode (such as aluminum electrode). The reverse cell consists of a cathode (generally ITO glass), an electron transport layer (generally oxide semiconductors such as zinc oxide, etc.), an active layer (composed of organic molecular such as organic conjugated polymers and fullerene derivatives, etc.), an electron transport layer (generally semiconductors such as molybdenum trioxide, etc.), and an anode (such as silver electrode). The active layer is obtained by blending the two materials, i.e., the donor and the acceptor, and solution processing or evaporation them, in which the organic conjugated polymer can serve as both the donor and the acceptor. In an ideal bulk heterojunction structure, the donor and the acceptor form an alternating co-continuous phase, which results in a microphase separation at a scale of tens of nanometers which not only can separate the excitons generated by optical excitation with high efficiency, but also can effectively transport the carriers after the exciton separation to the electrodes to generate the electric current (J. Peet, A. J. Heeger, G. C. Bazan, Acc. Chem. Res. 2009, 42, 1700).
In recent years, studies on the organic bulk heterojunction solar cells based on the solution processing of polymers have achieved remarkable results. In 2007, Prof Heeger et al. increased the power conversion efficiency of PCDTBT from 2.8% to 5.5% by controlling the morphology of the active layer with additives (J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, G. C. Bazan, Nat. Mater. 2007, 6, 497), and in the same year, a laminated device was prepared which obtained an power conversion efficiency of 6.5% (J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T-Q Nguyen, M. Dante, A. L. Heeger, Science 2007, 317, 222). Yu group of University of Chicago and Yang group of University of California at Los Angeles reported a series of polymers based on thienothiophene and benzodithiophene structures and results of more than 5% power conversion efficiency were obtained (Y. Liang, L. Yu, Acc. Chem. Res. 2010, 43, 1227). Moreover, for the first time a polymer bulk heterojuction solar cell with a power conversion efficiency of more than 7% was reported, in which PTB7 achieved a power conversion efficiency up to 7.4% (Y. Liang, Z. Xu, J. Xia, S-T. Tsai, Y. Wu, G. Li, C. Ray, L, Yu, Adv. Mater. 2010, 22, E135). Cao group increased the power conversion efficiency of PECz-DTQx from 4% to 6.07% using PFN modified electrodes, and recently increased the efficiency of a bulk heterojunction solar cell with an inverted structure to 8.37% which passed the certification by National Center of Supervision and Inspection on Solar Photovoltatic Products Quality (Z. He, C. Zhang, X. Huang, W-Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Adv. Mater. 2011, 23, 4636), and achieved the best results reported by current publications. Studies show that the efficiency of solar cells has close correlation to the mobility rate of polymers. Generally, the higher the mobility of the polymer, the higher of the efficiency of the solar cell (Chen, J.; Cao, Y. Acc. Chem. Res., 2009, 42, 1709). Therefore, increasing the mobility of the polymer has great significance on the studies on solar cells.
Organic conjugated polymers are a class of polymers obtained by polymerization of covalent bonds through conjugation from aromatic compounds. In order to ensure their good solubility and solution manufacturability, at least one solubilizing group needs to be introduced into at least one aromatic structure to increase their solubility. For example, the organic conjugated polymer as shown in the following formula:

wherein Ar1 and Ar2 are fragments of aromatic compounds, respectively; R1 and R2 are solubilizing groups introduced into the aromatic core Ar1, generally a group such as alkyl, halogen substituted alkyl, alkoxy, halogen substituted alkoxy, alkenyl, alkynyl, etc.; and n is the number of the repeating unit of the polymer, i.e., polymerization degree.
In primary studies (Lei, T.; Cao, Y; Zhou, X.; Peng, Y; Bian, J.; Pei, J. Chem. Mater 2012, 24, 1762.), we found that if a solubilizing group (such as an alkyl chain) is distributed in every one of the polymer units (as shown in FIG. 1(a)), it will affect the π-π stacking of the polymerization, thereby greatly affecting the mobility of the carriers in the polymer. This is because the van der Waals' radius between alkyl chains is 3.6-3.8 Å, while the distance of the π-π interaction is 3.4 Å (see the circle in FIG. 1(a) which indicates the repulsive effect of the alkyl chain against the aromatic group). As to this, we moved this alkyl chain from the smaller aromatic core Ar2 to the larger aromatic core Ar1, thereby increasing the mobility. On the other hand, traditionally a 2-branching alkyl chain (obtainable from Guerbet alcohol) is used as the solubilizing group (such as FIG. 1(b)) to avoid affecting the π-π stacking so as to achieve high mobility (Li, Y. Acc. Chem. Res., 2012, 45, 723; Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev., 2012, 112, 2208; Beaujuge, P. M.; Fréchet J. M. J. J. Am. Chem. Soc. 2011, 133, 20009; Wen, Y. Liu, Y. Adv. Mater 2010, 22, 1331; Chen, J.; Cao, Y. Acc. Chem. Res., 2009, 42, 1709). The purpose of the design is to ensure the π-π stacking while ensuring the solubility of the polymer.